1. Field of the Invention
This invention relates in general to test tube sealing caps of the type employed in centrifuge rotors and more particularly to a sealing cap for sealing the open end of a thin flexible test tube.
2. Description of the Prior Art
Analytical and preparative centrifuges are commonly provided with a rotor having a series of cavities, usually arranged in circular fashion, and adapted to receive test tubes for carrying a sample to be centrifuged. In a preparative centrifuge rotor, the axis of each cavity is angularly oriented with respect to the vertical rotational axis of the rotor so that the bottom of the test tube is further from the rotor axis than the top. An example of an angle head rotor is shown in FIG. 5 of U.S. Pat. No. 2,878,992 issued to Pickels et al. on Mar. 24, 1959 and assigned to the assignee of the present invention. Inclined-tube rotors are advantageous in that initially, precipitate is collected at the bottom of the tube at a more rapid rate than in a tube having its longitudinal axis perpendicular to the rotor axis were used.
During rotor operation, the sample is caused to climb up the outermost side of the tube wall and, eventually, if a sufficiently high rotational velocity is reached, the surface of the sample extends virtually parallel with the rotor axis. To prevent leakage of the sample from the tube, each test tube is provided with a cap sealing off the mouth thereof. It will be evident that if the test tube used in an angle rotor is initially full or almost full, leakage of the sample from the top of the test tube might occur unless the cap provides an adequate seal.
One form of centrifuge test tube cap assembly of the prior art includes a stem, a mating crown and a flat resilient washer, interposed between the stem and the crown. The stem is situated inside the upper part of the mouth of the test tube and the crown is dimensioned to fit snugly over the mouth. The stem and crown are coaxially coupled by suitable fastening means usually in the form of a crown-engaging nut threaded on a stud projecting from the stem. Between an upper surface of the stem and a lower surface of the crown is disposed a resilient washer which is compressed when the fastening nut is secured. Compression deforms the washer causing it to sealingly engage both the inner wall of the test tube and the stud projecting from the stem. The level of torque that is necessary to effect an adequate seal in such a test tube sealing cap is relatively high. In order to provide an effective seal, the tube must either be relatively stiff or the outer wall of the rotor cavity or a surrounding wall of the crown must be designed to support the centrifuge tube so that the resilient gasket produces a firm seal against the inner wall of the tube. When the centrifuge tube is relatively flexible, it is difficult to obtain a suitable seal in this fashion.
Thin wall centrifuge tubes rely to a certain extent on their contents for support. In lower "G" fields, the design of the capping system is based on the assumption that the volume of the tube cavity in the rotor and the volume of the tube contents are constant. In higher speed rotors which develop around half a million G's, the constant volume assumption is no longer valid. Under this high G loading, the rotor cavity stretches and increases its volume while the tremendous pressures on the fluid decreases its volume. This volume change of the sample fluid coupled with the volume increase in the size of the rotor cavity and tube allows the fluid surface to move down, depriving the tube of support at the upper end. If the tube cap assembly is not supported, in some manner other than by the tube, it may move down and sometimes results in a leakage around the cap. When the sample leaks from a tube which is flexible, it is likely to collapse. Therefore, it is necessary to design the tube cap assembly in such a manner that the tube cannot collapse into the rotor cavity when the tube leaks.
Accordingly, it is an object of the present invention to provide a new and improved tube cap assembly for a thin-walled flexible test tube.
It is a further object of the present invention to provide a test tube cap for a thin-walled relatively flexible test tube, which not only seals the mouth of the test tube, but also supports the tube within the rotor cavity and prevents collapsing of the tubes when the liquid level in the tube is lowered either by leakage or expansion under high G forces.
Further objects and advantages of the invention will become apparent as the following description proceeds, and these and other objects and advantages of the invention will become apparent from the following detailed description read in conjunction with the accompanying drawings.